我已经弄明白了这一点，但我找不到答案，并将张贴，因为有人可能会发现它有帮助。
下面是导致问题的代码：
models.py

class SModelQuerySet(models.QuerySet):
    def stubbed_outputs(self):
        return self.defer("outputs").annotate(
            has_outputs=models.Case(
                models.When(
                    models.Q(~models.Q(outputs={}) & models.Q(outputs__isnull=False)),
                    then=True,
                ),
                default=False,
                output_field=models.BooleanField(),
            )
        )

    def for_user(self, user):
        return self.filter(user=user)

class SModelManager(models.Manager):
    def get_queryset(self):
        return SModelsQuerySet(self.model, using=self._db).stubbed_outputs()

    def for_user(self, user):
        return self.get_queryset().for_user(user)

class OutputsManager(models.Manager):
    def get_queryset(self):
        return super().get_queryset().only("outputs")

    def for_user(self, user):
        return self.get_queryset().filter(user=user)

class SModel(models.Model):
    user = models.ForeignKey(User, on_delete=models.CASCADE, db_index=True)

    name = models.CharField(max_length=50)
    description = models.CharField(max_length=500, null=True, blank=True)

    inputs = models.JSONField(default=dict)
    outputs = models.JSONField(default=dict)

    objects = SModelManager()
    outputs_objects = OutputsManager()

    class Meta:
        verbose_name_plural = "scientific models"
        db_table = "smodels"
        managed = False

    def __str__(self):
        return self.name

views.py

class SharedModelViewSet(viewsets.ModelViewSet):
    permission_classes = (IsAuthenticated,)
    serializer_class = serializers.SharedModelSerializer

    @action(detail=True, methods=["post"])
    def save(self, request, pk=None):
        shared_model = self.get_object()
        model = get_object_or_404(
            models.SModel.objects.filter(user=shared_model.share_from),
            pk=shared_model.model_to_share_id,
        )
        model.pk = None
        model.user_id = shared_model.share_to.id
        model.save()

        shared_model.delete()

        return Response(
            serializers.SModelSerializer(model).data, status=status.HTTP_200_OK
        )

当这个视图试图保存时，我得到了这样的错误："ValueError: Cannot force an update in save() with no primary key."我无法弄清楚这一点，它是工作几个星期前。